1. Field of the Invention
This invention relates to an optical type encoder, and in particular to an encoder for forming interference fringes by diffracted lights from an optical type scale having a diffraction grating on which divisions are formed, and photoelectrically converting the interference fringes to thereby detect the displacement of the scale.
2. Related Beckground Art
In recent years, in precision machines such as NC machine tools and semiconductor printing apparatuses, there have been required precise displacement measuring apparatuses which can measure the displacement of a moving member such as a stage at the unit of 1 .mu.m or less (submicron). As measuring apparatuses which can measure displacement and amount of rotation at the unit of submicron, there are well known rotary encoders and linear encoders in which a coherent light beam such as a laser is used and interference fringes are formed by diffracted lights from an optical type scale mounted on a moving object and the interference fringes are photoelectrically converted to thereby measure the displacement of the scale.
Optical type encoders of this kind are numerously shown, for example, in U.S. Pat. Nos. 3,726,595, 3,738,753, 3,756,723, 3,891,321, 4,629,886, 4,676,645, Japanese Laid-Open Patent Applications Nos. 58-191906, 58-191907, 60-190812, 61-65165, 61-178613, 61-212728, 62-6119, and 62-12814.
Some of these prior-art optical type encoders are of the type in which a laser beam is projected onto an optical type scale having a diffraction grating formed thereon, diffracted light produced by the optical type scale is again directed to the optical type scale by reflecting mirrors and re-diffracted light produced by the optical type scale are utilized to form interference fringes. A signal obtained by photoelectrically converting the interference fringes thus provided by the re-diffracted lights is twice the signal obtained by photoelectrically converting the interference fringes provided by diffracted light obtained by one diffraction. Accordingly, the displacement of the moving member to be measured (the optical type scale) can be detected with double the resolving power.
However, whereas in the prior art, individual reflecting mirrors are provided for respective ones of the two diffracted lights produced in the optical type encoder and the respective diffracted lights are returned to the optical type encoder by these reflecting mirrors, a use of the plurality of reflecting mirrors has prevented the apparatus from being made compact. Also, when the components of the apparatus are all provided on the same side with respect to the optical type scale in order to increase the versatility of the encoder, the prior-art optical type encoders have become complex in construction.